The Milan exchange. (Milan, Gibson County, Tenn.) 1874-1978, 1887-05-14


gether an Opaque membrane, and there
fore, when brought into play by birds
flashing through the air, or flying
straight upwards in the full glare Of
the rays of the sun, it prevents the en-
trance of dust or other injurious mat
ter, protects the eye from too strong a
light, and at the same time does not
interfere with sight The motion OF
this membrane is performed in Curl
Gus manner; there are not two antago-
nistic muscles, one pulling forward
and the other backward, as
one might imagine, but only
one muscle, attached by a
tendon to the membrane. The nicti-
tating membrane itself is an elastic
substance, capable of being drawn out
by force like piece of elastic, and re-
turning to its former position when the
force is removed. The muscle which
exercises this force is one of the most
marvellous meohanisms to be found in
nature. Placed at the back of the eye,
it is passed through loop formed by
another muscle, and there infected as if
it were round pulley. This peculiar
ity, necessary for the rapid action
which is required, is one which has
many advantages. In order that the
membrane could be drawn over the
whole eye, longer muscle than could
be extended within the compass at the
base of the eye was required. A greater
length in smaller compass is obtained
by the cord of the main muscle making
an angle, and the whole action con-
trived to a nicely by the angle, instead
of being round fixed pivot, being
round loop formed by another muscle,
with the result, that whenever the sec
ond muscle contracts, it suddenly
twitches the first muscle at the point of
tendon, and thereby produces the rapid
action of the nictitating membrane.
One of the purposes of this men,
brane is, as mentioned, to wash
the eye. In man and some of
his nearest allies, this function is per-
formed by winking the eyelids, and
there is no necessity for any further as
sistance from third eyelid. In the
same way it is unnecessary in the in
habitants of the waters. 1t is there
fore absent in the great whale family
and in all fshes-the ocean, the river,
or the lake supplying the necessary To-
ton. But there is more than the abb
sence of this wonderful nictitating
membrane to notice in the eyes of
fishes, for they have no true eyelids at
4 all. Skin of structure sufficiently
transparent to allow of the passage of
the rays Of light passes over the eye,
and thus obviates the necessity for
even eyelids. In some cases, as in an
eel, a special protection to the eye is
found-the eel has to work its head
through hard, rough substances such
as gravel and sand. To defend the eye
from injury, therefore, a transparent,
horny, convex case is placed be-
fore the eye in such a manner as
10 defend the organ without imped-
ing the sight - another wonder
full instance of design. The con-
trivances for the protection of the eyes
in the different branches of the animal
world are indeed constant marvel to
the thoughtful. The nictitating men
brane is the most conspicuous, and
forms a contrast to the singular and
unique arrangement which exists only
in the chameleon. Instead of two eve.
lids the organ is covered by an cyclic
with a hole in it, and for an obvi
, Gus reason. The neck of the chame
leon is infeXible, and to make up fol-
this, the eye is so prominent that more
, than half the ball projects from the
| head, the muscle being such that the
7 pupil of the eye can be carried in every
) direction. To defend and lubricate
= the globe of an eye so unusually ex
7 posed, a special contrivance was neces
sary, hence the pierced lid-the liz
5 itself keeping the principal part of the
| surface of the eye under cover and in
: due state of humidity without con
l slant Meditation, and the aperture al
t lowing the necessary admission o
| light.
The position of the eyes on the head
: varying, as it does, the lower we de
= scend in the animal world, affords fur
+ Their food for reflection. In birds AM
5 the higher animals, the position is uni
: form, and obviously most beautiful
e symmetrical, and useful; but in fshe
| and insects we find an endless variet
/ as to position, direction and dimension:
S In some fishes, the eyes have an w
e ward aspect, and are very close tC
e gether; in others, they are at the side
: and so wide apart as to be slight
: downward in direction; whilst in sole.
9 turbots, founders and others of tn
same family, the eyes are placed, as
v were, one above the other, and bot
| u,,on tho Sa,na s;1a of tho hood CO..,